In an internal combustion engine, lubricating oil is heated and exposed to blowby gas during long-term operation of the engine, and unburned products of hydrocarbons and denatured products of oil additives are contained in the lubricating oil at the same time. These unburned products and denatured products of oil additives are generally collectively referred to as “oil sludge.” If such oil sludge adheres to engine components and is then deposited thereon, these components may be worn, and passages for the lubricating oil may be clogged, so that the functions of the engine components such as oil rings may be hindered.
FIG. 1 shows a cross-sectional view of an expander/segment oil control ring (three-piece oil ring) 10. This three-piece oil ring 10 includes a pair of annular side rails 11 each having an abutment joint and a spacer expander 12 that supports the side rails 11. Tabs 13 are formed on the inner peripheral side of the spacer expander 12, and protruding portions 16 for supporting the side rails 11 are formed on the outer peripheral side. Flat base dents 14 are provided in sections connecting the tabs 13 and the protruding portions 16. When the spacer expander 12 is combined with the side rails 11, spaces 15 surrounded by the tabs 13, the protruding portions 16, the base dents 14, and the side rails 11 are formed.
In the three-piece oil ring 10, the side rails 11 are pressed by radial and axial component forces caused by the angles of the tabs 13 of the spacer expander 12, thereby exerting a sealing function on the wall surface of a cylinder and on the upper and lower surfaces of a ring groove. In particular, a narrow-width three-piece oil ring having a reduced axial width, i.e., a reduced dimension h1, has good conformability to the cylinder wall surface and also has a side sealing function. Therefore, even when the tension of the oil ring is small, friction loss can be reduced without an increase in oil consumption. However, in such a three-piece oil ring 10, oil sludge is more likely to be deposited in the spaces 15 between the side rails 11 and the base dents 14 on the outer peripheral side of the tabs 13 of the spacer expander 12. In particular, when the three-piece oil ring has a reduced axial width, the deposition of oil sludge may cause sticking of the side rails 11 to the spacer expander 12. When sticking occurs, the conformability of the side rails 11 to the inner peripheral face of the cylinder is lowered, and the oil consumption is likely to increase.
To prevent adhesion and deposition of oil sludge onto an oil ring etc., it has been contemplated to use liquid repellent treatment with, for example, a fluorine-containing coating. More specifically, an oil repellent coating is formed on the surface of the oil ring to prevent adhesion of oil sludge in the lubricating oil. Examples of the material of the fluorine-containing coating used for the oil repellent treatment include polytetrafluoroethylene and fluoro alkylsilane. For example, Patent Literature 1 proposes a method of forming a liquid repellent film by a sol-gel method using a metal alkoxide and a fluoroalkyl group-substituted metal alkoxide prepared by substituting a fluoroalkyl group for part of the alkoxy group (alkoxyl group) in the metal alkoxide. It is known that a material containing a fluoroalkyl group has water repellency and oil repellency. Therefore, by providing a coating film having fluoroalkyl groups on its surface, liquid repellency is imparted to engine components to prevent adhesion and deposition of oil sludge. Patent Literature 2 discloses a technique for improving the effect of preventing adhesion and deposition of oil sludge by using a fluorine-containing coating having an increased thickness. To increase the thickness of the coating, the polymerization of the fluoroalkyl group-substituted alkoxide is promoted before a coating solution is applied to a substrate.
Patent Literature 3 describes that, when components for an internal combustion engine are coated with a carbon-based film having a prescribed surface free energy and a prescribed coating roughness, repellency to deposits (oil sludge) is improved. Therefore, deposition of the deposits and sticking are suppressed, and efficient combustion operation is maintained with small performance degradation. Polypropylene resins, perfluoroethylene propylene (FEP) resins, polytetrafluoroethylene (PTFE), fluoro alkylsilanes, etc. are exemplified as the carbon-based film.
In a multi-piece oil ring described in Patent Literature 4, a nitride layer is formed on the shear surfaces of the tabs of a spacer expander, and a Ni plating is formed on the surfaces of the spacer expander other than the shear surfaces. In addition, a thin film formed only of a fluorine-based organic material is formed on the outermost surfaces of the spacer expander that face the side rails and/or the surfaces of the side rails that face the spacer expander. It is shown that the formation of the Ni plating prevents nitriding of the surfaces other than the shear surfaces of the tabs, so that unevenness in tension of the spacer expander that is caused by unevenness in thickness of the nitride layer is reduced. In addition, there is a description that the thin film coating formed only of the fluorine-based organic material provides the effect of preventing adhesion of oil sludge. Specific examples of the raw material of the fluorine-based organic thin film include fluoroalkyl group-substituted metal alkoxides in which part of the alkoxy groups are substituted with fluoroalkyl groups and metal halides having fluoroalkyl groups.
As described above, to prevent adhesion and deposition of oil sludge, it has been contemplated to coat the surface of an oil ring with a fluorine-containing coating. However, with the conventional coating structures, the effect of preventing adhesion and deposition of oil sludge is not sufficient. Therefore, there is a demand for a multi-piece oil ring for an internal combustion engine that can prevent sticking even after long-term operation and can maintain a high oil-control function.    [Patent Literature 1] Japanese Patent Application Laid-Open No. 2000-27995    [Patent Literature 2] Japanese Patent Application Laid-Open No. Hei. 10-157013    [Patent Literature 3] Japanese Patent Application Laid-Open No. 2006-291884    [Patent Literature 4] Japanese Patent Application Laid-Open No. 2006-300224